The Thule Xadapt 2 is an optional kit that makes it possible to use Thule roof mount bike carriers 599XTR, 724, and 725 on larger crossbars. The Xadapt2 works on Thule Rapid Aero and Aeroblade aluminum load bars, Thule Xsporter and Professional truck racks, and Rhino-Rack bars. Xdapat2 includes replacement spacers and bolts that allow the mounting plates to fit around larger diameter roof rack cross bars. If you have a roof rack with square load bars, you do not need this adapter kit. Manufacturer's part number: XADAPT2